Description of Twitter bootstrap import into Moodle

Twitter bootstrap
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This theme uses the original unmodified version 4.0.0-alpha-3 Twitter bootstrap sass files.
The bootstrap repository is available on:

https://github.com/twitter/bootstrap.git

To update to the latest release of twitter bootstrap:
* remove all files from scss/bootstrap,
* download the new scss files and store them in scss/bootstrap
* update ./thirdpartylibs.xml